1. Field of the Invention
The present invention relates to a vacuum device and a fabrication method thereof and more particularly, to a vacuum device having a first terminal from which electrons can be emitted and a second terminal into which the electrons flow, and a fabrication method thereof.
2. Description of the Related Art
Recently, vacuum devices of this sort have been developed, which are fabricated by utilizing micro processing techniques generally used in the field of semiconductor devices. The vacuum device is different from a conventional vacuum tube in that the device utilizes a cold cathode. That is, in the conventional vacuum tube, a filament is heated so that electrons are thermally excited to be emitted therefrom into a vacuum space. On the other hand, in the vacuum device, no filament is provided, but a cathode having a particular configuration is provided so that a large electric field can be generated around the cathode, thereby causing electrons to be emitted from the cathode. Generally, the cathode is pyramidal shaped and the edge thereof is sharp. The sharpness of the edges is important to the device characteristics and their radius of curvature should be several hundreds angstroms or less.
Conventionally, two methods of forming the cathode are known, one of which was developed by Spindt et al., Stanford Research Institute. In the method, an aluminum film is formed on a rotating insulator substrate which is supported obliquely by the vacuum evaporation technique thereby forming a sacrificial layer having a visor and thereafter, a metal having a high melting-point such as molybdenum is deposited on the sacrificial layer. Then, the aluminum sacrificial layer is removed to obtain a pyramidal shaped and sharp-edged cathode (see J. Appl. Phys. Vol. 39, pp3504, 1968).
The other method was developed by Gray et al, Naval Research Laboratory, in which a pyramidal shaped and sharp-edged cathode is obtained by utilizing the anisotropic etching technique of silicon (see IEDM Tech. Dig., pp776, 1986). The processes of the method are shown in FIGS. 1A to 1E.
First, a single-crystal silicon substrate 72 is prepared and a silicon nitride film 71 is formed thereon by the Chemical Vapor Deposition (CVD) technique, as shown in FIG. 1A. Then, as shown in FIG. 1B, the nitride film 71 is removed except for a portion in which a sharp-edged pyramidal cathode is to be formed, and the substrate 72 is anisotropically etched by using an anisotropic etching solution such as hydrazine, ethylene diamine or the like. When a predetermined time is passed, the etching of the substrate 72 is stopped thereby a pyramidal portion 73 being obtained, as shown in FIG. 1C. Gray et al was utilized the portion 73 thus obtained.
Next, to make the pyramidal portion 73 more sharp-edged, the substrate 72 is thermally oxidized to form an silicon oxide film 74 thereon. As a result, the pyramidal portion 73 of silicon is made more sharp-edged, as shown in FIG. 1D.
To make a gate, a silicon oxide film 76 is deposited on the silicon oxide film 74 and the silicon nitride film 71 shown in FIG. 1D by the CVD technique and then, a molybdenum film 75 is formed on the film 76 by the vacuum evaporation technique, the state of which at this time is shown in FIG. 1E.
Finally, the nitride film 71, the oxide film 76 and the molybdenum film 75 placed on the top of the pyramidal portion 73 are removed by the lift off method thereby the surface of the portion 73 being exposed. Thus, the vacuum device is obtained. The oxide film 76 and the molybdenum film 75 remain except those on the portion 73. The remained portion of the film 75 acts as a gate.
With the above-described method using the anisotropic etching technique of silicon, there are problems in that it is very difficult to obtain the sharp-edged pyramidal portion with a reduced dispersion and that the prior art requires complex technology for performing the method.
In addition, there is another problem that the positions of the cathode and gate cannot be set with accuracy and without restraint.